In the monitoring of long term ECGs for features indicating intermittent heart irregularities, syncopal events and the like, minimally invasive monitors like the Reveal (TM) electrocardiogram event recorder manufactured by Medtronic, Inc. have proven to be useful, and now appear to be accepted by a segment of the medical community for use in diagnosing patient problems like fainting. However, particularly when the device employs automatic arrhythmia detection triggers to activate the storage of a segment of the ECG, the presence of noise in the ECG signal channel may trigger activation's inappropriately, causing the memory to become full of unwanted portions of the cardiac electrogram that may be of little to no use in diagnosing the patient condition.
Accordingly, we have developed a method along with apparatus to employ it, for eliminating a high proportion of the noise in the signal which might otherwise cause improper activation of automatic electrocardiogram (ECG) storage.
Monitoring can be done using implantable pulse generators such as pacemakers and other heart stimulating devices or devices with leads in the heart for capturing physiologic parameters, including the ECG. However, the expense and risk from implanting a pacemaker or changing out one without these functions is something both patients and physicians would prefer to avoid. Such devices, in addition to performing therapeutic operations, may monitor and transmit cardiac electrical signals (e.g., intracardiac electrograms) to an external diagnostic device typically with leads fixed in the patient's heart, to observe electrical activity of a heart. It is common for implanted cardiac stimulation devices to send intracardiac EGM (electrocardiograms taken from in the heart) signals to a monitoring device, such as an external programmer, to allow a user to analyze the interaction between the heart and the implanted device. Often the user can designate that the communication from the implantable device to the programmer include a transmission of codes which signal the occurrence of a cardiac event such as the delivery of a stimulation pulse or a spontaneous cardiac depolarization.
For example, U.S. Pat. No. 4,223,678, (incorporated herein by this reference in its entirety) entitled "Arrhythmia Recorder for Use with an Implantable Defibrillator", issued to Langer et al. on Sep. 23, 1980, discloses an arrhythmia record/playback component within an implantable defibrillator. ECG data is converted from analog to digital (A/D) form and stored in a first-in, first-out memory. When the defibrillator detects an arrhythmia event, it disables the memory so that no further ECG data is recorded in the memory until a command is received from an external monitoring device. This command requests the implantable defibrillator to transmit the stored ECG data to the monitoring device via telemetry. Langer et al. in U.S. Pat. No. 4,407,288, (also incorporated by reference herein) entitled "Implantable Heart Stimulator and Stimulation Method", issued Oct. 4, 1983, discloses a programmable, microprocessor based implantable defibrillator which senses and loads ECG data into a memory via a direct memory access operation. A processor analyzes this ECG data in the memory to detect the occurrence of an arrhythmia event afflicting a patient's heart. Upon such an event, the defibrillator may generate a therapy to terminate the arrhythmia event and store the ECG data sequence of the event, for transmission to an external monitoring device and later study. In normal circumstances, when no arrhythmia event is occurring, the defibrillator continuously overwrites the ECG data in the memory.
U.S. Pat. No. 4,556,063, (too, incorporated herein by this reference) entitled "Telemetry System for a Medical Device", granted to D. L. Thompson et al, 1985, teaches a pulse interval telemetry system capable of transmitting analog data, such as sensed intracardiac electrogram signals, without converting analog data to a digital numeric value. The Thompson et al. telemetry system is capable of sequentially transmitting both digital and analog data, individually and serially, in either an analog or a digital format, to a remote receiver. The features and capabilities of these pacemaker/defibrillator devices is now well known, but the problems in long term monitoring for events and adequate recordation remain.
Other background includes an article in the December 1992 Vol. 15 edition of PACE (15:588), a feasibility study for implantable arrhythmia monitors and reported by Leitch et al. Subcutaneous, Bipolar "Pseudo-ECG" Recordings using an Implantable Monitoring System and at chaired poster presentation of the North American Society of Pacing and Electrophysiology (NASPE).
Further, a leadless implantable sensor for cardiac emergency warning was described in U.S. Pat. No. 5,404,887 issued to Knowlan et al. which detects heart events through impedance measurement sensed using a coil. See also Yomtov et al, U.S. Pat. No. 5,313,953 (incorporated herein by this reference) which describes (in FIG. 26) a large but leadless implant.
With sufficient hardware and connections to the body, numerous other physiologic parameters may be sensed as is pointed out in U.S. Pat. No. 5,464,434 issued to Alt and U.S. Pat. No. 5,464,431 issued to Adams et al. (both also incorporated herein by this reference).
Nevertheless there is still a need to reject the noise inherent in the ECG signals for automatic detection of arrhythmias. It is particularly problematic to detect abnormal ECGs taken in the "far field" of the subcutaneous electrodes used in the Reveal(TM) and other under-the-skin long term monitors.
While `blanking` and `refractory` periods are commonly used in pacemakers and implantable cardiodefibrillators (ICD's) today, their application to implantable devices which do not deliver therapy, and to ECG storage automatic triggers and their use in far field ECG recording has not been seen. Such blanking and refractory periods do eliminate or limit the sensing abilities of the implanted medical devices in which they are used, however. Examples of such periods in the therapy delivery device art include U.S. Pat. No. 5,759,196, Hess, et al., U.S. Pat. No. 5,188,105, issued to Keimel, and U.S. Pat. No. 4,974,589, issued to Sholder. All these patents are incorporated by this reference in their entireties by this reference thereto. Nevertheless, none of these use such periods to exclude signals from the far field ECG, which they do not have the capacity to record.
Picking up good ECG signals in the far field but within the body is fraught with noise problems. (To clarify, by "far field" we mean ECG measurements taken outside the heart but under the skin. This commonly may also be called subcutaneous ECG measurement.)
Particularly in looping ECG recording systems which automatically detect arrhythmias according to specific arrhythmia detection criteria and retain segments of the ECG in recorded data memories as well as in other ECG recording systems, there are several areas in which false detects can fill up the memory for data storage with unwanted data. (`False detects` here means that a predetermined number of QRS segments or R-waves has been detected over an appropriately predetermined trigger time to set off a trigger criterion that is monitoring the detection of R-waves and sending the data to the trigger monitoring circuit. The trigger circuit then sets off a detect signal, forcing the implantable recorder to record a segment of the ECG into the memory of the implantable medical device).
First among likely noise sources is false detection's of noise leading to false Tachy detects (i.e. inappropriate detections of an electrocardiogram segment). Muscle noise (EMG noise) can easily dominate the signal of the Electrocardiogram. While it is impossible to filter out all of this noise and it is particularly susceptible to noise in the subcutaneous area where the leads of the small ECG implantable monitor are usually located. This noise will generally be broad band and subsumes the band of the standard recorded ECG. The standard recorded ECG band is -3 dB band with ranges from 0.1 Hz to 32 Hz.
A problem caused by noise is the overreaction of the recording system such that because of false repeat detections of the same sequence of arrhythmia, the memory overfills with segments of the same event. Such a problem is expected in Looping ECG recording systems which automatically detect an arrhythmia and then saves data that has been monitored for a period of time previous to the detection, as well as data from a period of time following the detection. We can remove some of this with our removal of noise, and some of this problem can be overcome by the use of redundancy in the trigger itself. (for example of redundancy, a string of 12 or 16 Tachy sized R to R intervals may be required).
A third source would be from electrical interference from other electrical devices in the area (EMI noise). Commonly this is in the 60 Hz range because most alternating current and devices run on this frequency. Any commonly available filtering techniques and digital signal processing techniques may be employed beyond what is described here for reducing this particular kind of noise.
A fourth cause of false detects may come from wide QRS complexes. These could include an inappropriate Tachyarrhythmia detection for the occurrence of a very long QRS segment caused perhaps by a patient with congestive heart failure if, for example, the tachy automatic trigger was short enough to register within a long QRS and the presence of high amplitude signals with noise caused multiple detections within that long QRS. (Thus, where the R-wave detector finds several putative R-wave detections during a single wide QRS, and the next QRS which also has several possible R-wave detections in it, a very short string of such events will cause a tachyarrhythmia event trigger to fire).